Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds including, among other things, the oxides of nitrogen (NOx). In order to reduce NOx emissions into the atmosphere, some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR). SCR is an exhaust treatment process where a reductant, most commonly urea ((NH2)2CO) or a water/urea solution, is selectively injected from an onboard supply into the exhaust gas stream of an engine. The injected urea solution decomposes to form ammonia (NH3), HCO, and H2O, and the NH3 is adsorbed onto a downstream substrate, often referred to as an SCR catalyst. NH3 that is adsorbed by the SCR catalyst reacts with NOx in the exhaust gas to form water (H2O) and diatomic nitrogen (N2).
When exhaust temperatures are relatively low, such as after starting an engine that has cooled (sometimes referred to as a “cold start” condition), the injected urea solution does not fully decompose to NH3 before reaching the SCR catalyst. As a result, the SCR catalyst can become clogged with urea deposits, thereby reducing its ability to store NH3 and reduce NOx emissions. Moreover, any remaining NH3 stored on the SCR catalyst may be insufficient to adequately reduce NOx emissions.
One attempt to reduce NOx emissions at relatively low temperatures is disclosed in U.S. Pat. No. 8,621,847 issued to Gonze et al. on Jan. 7, 2014 (“the '847 patent”). Specifically, the '847 patent discloses an engine exhaust system having an SCR catalyst and an air pump that directs ambient air to the SCR catalyst and to the exhaust manifold of the engine. A controller determines when the engine is shut off and actuates the pump to cool the SCR catalyst with the ambient air to a temperature at which its NH3 storage capacity is increased. The controller then diverts the ambient air to the exhaust manifold and forces it through the exhaust system where it absorbs heat from the warm exhaust system components. Urea is injected into the warmed air and decomposes into NH3 before being adsorbed on the now cooled SCR catalyst for later use under cold start conditions.
Though perhaps somewhat effective at increasing the NH3 storage capacity of an SCR catalyst, the exhaust system of the '847 patent may not be practical in various applications. Particularly, the additional air pump and air conduit disclosed in the '847 patent may not be feasibly integrated into an exhaust system having strict packaging constraints. Additionally, the process of storing NH3 in the SCR after the engine has stopped may be inefficient. Further, the SCR catalyst may not be optimally designed for storing NH3 under varying conditions.
The exhaust system of the present disclosure addresses one or more of the needs set forth above and/or other problems of the prior art.